The use of a touch input device disposed over the viewing surface of a computer display to provide a "user friendly" means for the control of a data processing system is well known in the art. For example, U.S. Pat. No. 5,016,008 to Gruaz et al. describes a touch-sensitive pad that is used to input information regarding the position of a pointer on a video screen of a computer. Also, U.S. Pat. No. 5,025,411 to Tallman et al. describes a touch-sensitive screen on an oscilloscope for allowing the user to select menu options with a touch of a finger. These devices are designed to allow an unsophisticated user to perform desired tasks on a computer system without extensive training. Human factor studies have shown that an input device which allows the user to input the data directly on the computer display achieves greatest immediacy and accuracy between man and machine. Such a device is generally known in the art as a touch input device.
In the current graphical user interfaces developed to aid man/machine interaction, there are many items, such as menu selections, icons or windows which a user can most easily select by the touch of a finger. Other means of selecting these items are known to the art, either with a mouse pointing device, or with a light pen, which is a pointing device developed for use on cathode ray tubes (CRT monitors) as disclosed in U.S. Pat. No. 4,697,175 to Macdonald and Japanese Laid-Open Patent Application No. 01304587. In other advanced software applications which perform freehand drawing, gesture or motion recognition or handwriting capture, a stylus is more effective because of its greater precision. U.S. Pat. No. 4,754,267 to Reynolds et al., U.S. Pat. No. 4,814,760 to Johnston et al., and U.S. Pat. No. 5,007,085 each disclose a stylus used as a pointing device on a tablet or workpad for input to a computer system. Thus, it would be convenient to utilize a touch input system which allows both stylus and finger touch detection. One such system is described in commonly assigned U.S. Pat. No. 4,686,332, to Greanias et al., entitled "Combined Finger Touch and Stylus Detection System For Use On The Viewing Surface Of A Visual Display Device", and which is hereby expressly incorporated herein by reference.
Unlike a mouse pointing device where the user is not necessarily interested in the path which has taken the mouse pointer to its current position, a stylus with a touch input device is generally used to generate input information resembling handwriting or freehand drawing. U.S. Pat. Nos. 4,177,354 to Mathews, 4,277,783 to Sampieri et al., 4,550,438 to Convis et al. and 4,972,496 to Sklarew all describe attempts to simulate handwriting or freehand drawing using a pointing device. As the user makes strokes on the touch input device, there is displayed an "ink" trail which simulates the strokes made by a pen on paper. The ink trail thus provides feedback and aids the user in directing the stroke. U.S. Pat. No. 4,675,665 to Halliwell describes computing the positional data of the stylus and updating the ink trail display after each refresh cycle of the display screen. Such stylus-driven touch input devices typically generate a large number of points per second (p/s), which has the effect of creating a high CPU overhead due to the frequent invocation of the inking software subroutine. Due to the high CPU overhead, it is not unusual for the ink trail to lag far behind the tip of the stylus since the points are generated faster than the software can ink them.